Coordination Mode Dependent Excited State Behavior in Group 8

These data support a PT3 ligand based lowest excited state in the case of both P ... data is made available by participants in Crossref's Cited-by Lin...
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Coordination Mode Dependent Excited State Behavior in Group 8 Phosphino(terthiophene) Complexes Stephanie A. Moore,† Jeffrey K. Nagle,‡ Michael O. Wolf,*,† and Brian O. Patrick† † ‡

Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada Department of Chemistry, Bowdoin College, Brunswick, Maine 04011, United States

bS Supporting Information ABSTRACT: The ground and excited state behavior of four Ru(II) and Os(II) bipyridyl complexes containing the 30 -(diphenylphosphino)2,20 :50 ,200 -terthiophene (PT3) ligand in two different coordination modes (P,S and P,C) is reported. The complexes are generally stable under extended photoirradiation, except for [Ru(bpy)2PT3-P,S](PF6)2 which decomposes. Emission lifetimes and transient absorption spectra and lifetimes have been obtained for all the complexes. These data support a PT3 ligand based lowest excited state in the case of both P,S bound complexes, and a charge separated lowest excited state in both P,C bound complexes, conclusions supported by Density Functional Theory (DFT) calculations.

’ INTRODUCTION Group 8 polypyridyl complexes have been widely studied as photosensitizing dyes for solar energy harvesting because of their chemical stability, absorbance, and emission properties, and their ability to participate in electron and energy transfer processes.1,2 In the dye-sensitized solar cell (DSSC) pioneered by Gr€atzel and O’Regan,3 electron injection into a wide bandgap semiconductor from such dyes is central to absorption of solar light and charge separation. Conjugated oligomers and polymers are another class of materials that are being investigated for application as light absorbers in organic solar cells.4,5 The efficiency of cells based on these materials is typically limited by exciton recombination on the conjugated chains.6 A different approach involves coupling a light absorbing metal dye to a conjugated backbone, with the goal of exciting an electron from the conjugated group to a peripheral ligand on the metal complex generating a charge-separated excited state. This approach offers the possibility of hole transport over longer distances via the π-conjugated backbone. However, energy transfer to competing, low-lying states can present complications.7,8 When metal complexes are coordinated to conjugated oligomers and polymers, interactions occur which modify the physical, chemical, and electronic properties of both species.9 Metals can be incorporated into conjugated materials, such as oligothiophenes or polythiophenes, by direct insertion into the chain,9,10 direct bonding to the backbone through a thiophene,11 bipyridyl,12 or other group, or as pendant groups attached directly through a ligand.13 The coordination mode can have a significant effect on the excited state interactions. We have previously shown that the ground and excited state behavior of r 2011 American Chemical Society

Chart 1

the Ru(II) bis(bipyridyl) complexes14,15 1 and 2, where a terthiophene is tethered to the metal through a diphenylphosphine ligand (Chart 1) via either P,C or P,S coordination, differ significantly from each other. Visible light excitation of an analogue of complex 2, in which the conjugation in the phosphino(oligothiophene) ligand is extended to five thiophene rings, resulted in a transient species assigned as a chargeseparated excited state.16 Others have recently explored the utility of cyclometalated complexes as dyes in DSSCs,17,18 raising the possibility that cyclometalated complexes such as 2 may find application in these types of cells. The possible presence of low-lying metal-centered (MC) excited states8 or low-lying Received: February 24, 2011 Published: May 02, 2011 5113

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Scheme 1

Figure 1. Solid state structures of [Os(bpy)2PT3-P,S](PF6)2 (3) (left) and [Os(bpy)2PT3-P,C](PF6) (4) (right). Hydrogen atoms, counterions, and solvent in lattice removed for clarity. Thermal ellipsoids are drawn at 50% probability.

ligand-based triplet states7 which can be populated from the metal-to-ligand charge transfer (MLCT) state may lead to alternate routes for deactivation of the excited state in these Ru complexes. Os(II) has a larger ligand field splitting energy (by about ∼30%) than Ru which results in an increased energy gap between MLCT and MC states,8 thus Os(II) analogues of the Ru(II) complexes 1 and 2 may have longer charge-separated lifetimes. On the other hand, some Os(II) polypyridyl complexes have been shown to have shorter excited state lifetimes than the analogous Ru(II) complexes because of enhanced spinorbit coupling.19 In cases where ligands with a triplet state close in energy to the 3MLCT state are present, energy transfer between the two states may occur.20 Os(II) polypyridyl complexes also typically have lower energy MLCT states, leading to broader absorption of light over more of the visible spectrum. This is an advantage for solar energy harvesting where low energy photons must also be captured for maximum efficiency. Here, we report two new Os(II) complexes (3 and 4) with conjugated terthiophene based ligands, and compare their photophysical and excited state electronic properties to the Ru(II) analogues.

’ RESULTS AND DISCUSSION Synthesis and Structure. The P,S-Ru(II) complex 1 was previously prepared by dechlorination of Ru(bpy)2Cl2 with silver tetrafluoroborate followed by addition of 30 -(diphenylphosphino)-2,20 :50 ,200 -terthiophene (PT3) and precipitation of the

product as a hexafluorophosphate salt. The brown [Ru(bpy)2 PT3-P,C](PF6) complex (2) was obtained by reaction of 1 with base. Initially, we attempted to prepare the corresponding Os complexes using the same route, but the poor solubility of the (NH4)2OsCl6 starting material prevented successful isolation of the product 3. Variation of the volume of solvent used, the time at reflux, and deletion of the filtering step did not help, and in all cases we were unsuccessful in isolating the desired Os analogue of 1. Meyer and co-workers have previously reported that the use of Ag salts for dechlorination of Os complexes does not work well, and recommended heating the metal precursor and ligand together in high boiling solvents such as glycerol or ethylene glycol.21 In our hands, this method yielded only a small amount of the desired product along with multiple other products, including the monochloro species according to mass spectroscopic analysis. Longer heating at reflux (up to 5 days) did not improve the yield. Further modifications to the conditions established that using a solvent mixture of ethanol and water, in a two-to-one ratio, with extended (3648 h) heating at reflux, gave the desired Os complex 3 in 42% yield (Scheme 1). The product was purified by column chromatography on neutral alumina. A similar procedure to that used to obtain 2 was employed to obtain the brown cyclometalated species [Os(bpy)2PT3-P,C](PF6), 4, in 61% yield. Single crystals of both complexes 3 and 4 were grown from solution, and solid state structures of both complexes were obtained (Figure 1). The bonding arrangement in both new 5114

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Figure 2. ADF-calculated plots of some frontier molecular orbitals (one of each spinorbit pair shown, all a1/2 in C1 symmetry) for (a) 3 and (b) 4, together with calculated energies (eV) for selected orbitals, electron occupations, and Mulliken metal atomic orbital percentages. The lowest energy orbital (453a1/2 for both complexes) plot is lower left in both cases, alternating from one side to the other with increasing energy so that the HOMO plot is on the left side of the energy levels and the LUMO plot is on the right side of the energy levels in both cases.

structures is identical to that in the Ru analogues, and some differences in bond lengths and angles are observed (Supporting Information, Table S1). The S1C36C37S2 torsion angle of 154.9(3)° in 3 is somewhat larger than in 1. The larger torsion angle indicates increased coplanarity, resulting in increased πorbital overlap between adjacent thienyl rings. In 3, the bound thiophene ring is tilted away from the OsS bond at an angle of 59.7°, slightly larger than the tilt angle in 1 (58.3°). The thiophene tilts away from the metalsulfur bonds in these compounds to reduce unfavorable π*-antibonding interactions between the thiophene and the metal.22 The Os center may result in a greater degree of antibonding interaction, and consequently a larger tilt angle. The OsC bond in complex 4 was found to be 2.095(3) Å, which is comparable to the metal carbon bond length in the Ru analogue, as are the other bond lengths. The S1C36C37S2 torsion angle indicates the two locked thiophene rings in 4 are more coplanar than in 2. The thiophene ring is almost coplanar with the vector of the OsC bond (tilted 5.9°), a smaller tilt angle relative to the Ru analogue (7.3°). Here, the bound carbon is sp2 hybridized, and the tilt angle suggests antibonding interactions may also play a role in these complexes. Density Functional Theory (DFT) Calculations. Orbital plots for the Os(II) complexes together with their energies, electron populations, and calculated metal contributions are shown in Figure 2. Spinorbit effects were included in the calculations so that all orbitals are listed as a1/2 pairs. The results reveal many similarities in bonding between the corresponding Ru and Os complexes (orbital plots for the Ru analogues shown in Supporting Information, Figures S5 and S6). In comparing the

corresponding PT3-P,S and PT3-P,C complexes, it is found that for the two P,S complexes both the highest occupied molecular orbital (HOMO) and the HOMO-1 have no metal contributions while the P,C complexes have minor metal character for the HOMO (3% Ru for 2 and 10% Os for 4) and nearly equal metal and PT3 contributions for the HOMO-1 (56% Ru for 2 and 43% Os for 4). DFT calculations on all four complexes indicate that the total metal bond order calculated by five different methods (Mayer, GophinatanJug, and three NalejawskiMrozek definitions) is greater by about 0.6 for 3 vs 1 and 4 vs 2, indicating stronger overall ML bonding in the Os complexes compared to the corresponding Ru complexes. The increase in the total metal bond orders of 0.3 for both 2 vs 1 and 4 vs 3 indicates stronger overall ML bonding in the P,C coordination mode where the PT3 ligand is formally deprotonated, compared to the P,S mode where the ligand is formally uncharged. This is not surprising given the lower positive charge in complexes 2 and 4 which arises because of a negative formal charge on the coordinated C atom in the deprotonated PT3 ligand. The significantly larger MC bond orders for 2 and 4 compared to 1 and 3 are consistent with this conclusion. There is little difference in the calculated MP and MN (N trans to P) bond orders between complexes 1 and 2, and between complexes 3 and 4. The values of the total metal bond orders are provided in Supporting Information, Table S3 and range from 4.2 ( 0.4 for 1 to 5.0 ( 0.5 for 4 depending on the method used to calculate them. The Hirshfeld, VDD, Mulliken, and MDC-q methods of calculating atomic charges on the metals all result in higher 5115

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Inorganic Chemistry positive charges by 0.10.2 for 1 vs 3 and 2 vs 4, while the opposite is found with the AIM and NPA methods (Supporting Information, Table S4). The higher metal atomic charges for the Ru complexes compared to the corresponding Os complexes calculated by all but the AIM and NPA methods are consistent with a slightly higher electronegativity for Os compared to Ru,23 and in fact the electronegativity equalized charges24 calculated for the Ru complexes are higher by 0.1 than those for the corresponding Os complexes. All six ADF methods of calculating atomic charges on the metals show an increase in positive charge for 2 vs 1 and 4 vs 3. The unexpectedly larger charges for the complexes with the P,C coordination mode suggest greater M-bpy π back-bonding and/or less bpy-M σ donation compared to those with the P,S mode. The more negative charges on the N atom trans to the coordinated C atom in 2 and 4 compared to those for the N atom trans to the coordinated S atom in 1 and 3 are consistent with this conclusion. The longer MN bond distances for the N atom trans to C in 2 and 4 (2.15 and 2.12 Å, respectively) compared to the MN bond distances for the N atom trans to S in 1 and 3 (2.07 Å for both complexes) suggests less bpyM σ donation in 2 and 4 compared to 1 and 3. Cyclic Voltammetry. The cyclic voltammogram of 3 (Supporting Information, Figure S7a) shows a quasi-reversible oxidation peak at 1.23 V vs SCE, ∼ 0.25 V lower than the first oxidation peak of 1. ADF-calculated Mulliken AO contributions indicate that both the HOMO and HOMO-1 orbitals of 1 and 3 are completely localized on the PT3 ligand. The first reduction peak of 3 occurs at 1.36 V vs SCE and is irreversible. This reduction is presumably localized on the bpy ligands, consistent with ADF calculations that reveal neither the lowest unoccupied molecular orbital (LUMO) nor the LUMOþ1 have more than a 5% Os AO contribution. The cyclic voltammogram of 4 (Supporting Information, Figure S7b) shows two reversible waves, at 0.28 and 0.95 V vs SCE. Some small features are observed in the voltammogram and are attributed to products resulting from scanning over the full potential range. For 4, the ADF-calculated HOMO is about 90% PT3 localized while the HOMO-1 (0.48 eV lower in energy) is nearly equally distributed between the Os and PT3 units, suggesting the oxidation wave at 0.28 V to be PT3-localized and the 0.95 V oxidation wave to be due to removal of an electron from a mixed Os-PT3 orbital. These assignments differ from those previously reported for the Ru analogues.15 The earlier assignments were based only on the observation of small shifts in potential with methyl substitution on the PT3. The oxidation potential of 0.95 V for 4 is 0.16 V lower than the corresponding value for 2 and is consistent with the smaller calculated atomic charge for the metal in 4 relative to 2. In addition, 4 has reduction waves at 1.45 and 1.75 V vs SCE, both presumably corresponding to bpy-based processes as indicated by the ADF-calculated AO parentages for the LUMO and LUMOþ1. These values are 0.08 and 0.03 V less negative, respectively, than the corresponding values for 2 and are consistent with the 0.06 V lower calculated LUMO energy for 4 versus 2. The substantial decrease in the Os2þ/3þ oxidation potential between 3 and 4 is consistent with the lower energy of the mixed metalligand to ligand charge transfer transition (MLL0 CT) transition (see below). A similar reduction in the Ru2þ/3þ oxidation potential was also observed between complexes 1 and 2,14 and can be accounted for at least in part by the reduction in overall charge from þ2 for 1 and 3 to þ1 for 2 and 4. Ground State Absorption Spectra. The UVvis absorption spectra of the Os complexes in CH3CN are shown in Figure 3,

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Figure 3. UVvis absorption spectra of (a) [Ru(bpy)2PT3-P,S] (PF6)2 (1), [Os(bpy)2PT3-P,S] (PF6)2 (3) and (b) [Ru(bpy)2PT3-P,C] (PF6) (2) and [Os(bpy)2PT3-P,C] (PF6) (4) in CH3CN.

along with the spectra of the Ru analogues for comparison. The absorption spectrum of complex 3 contains two major bands, a band at 286 nm assigned to the π f π* transition of the bipyridine group, and a lower energy band with λmax = 394 nm. The low energy band is shifted only very slightly from the corresponding band in the spectrum of 1, similar to observations of the spectra of tris(bipyridine) metal complexes, where the 1MLCT band in [Ru(bpy)3]2þ has λmax = 451 nm25 and λmax = 450 nm in [Os(bpy)3]2þ with the 3MLCT band observed between 520 and 700 nm.26 This lower energy band in 1 was assigned to a charge transfer transition involving a mixed metal/ terthiophene HOMO and bipyridyl based LUMO (a 1MLL0 CT transition).15,16 ADF calculations for 1 and 3 indicate that the HOMO and HOMO-1 have no metal atomic orbital contributions, whereas the HOMO-2, -3, and -4 orbitals all contain substantial metal contributions and all lie within about 1 eV of the HOMO in energy. Therefore for both 1 and 3 the broad, overlapping absorption bands above 350 nm are likely to contain orbital contributions from both PT3-localized and M-PT3 delocalized orbitals and are best considered as 1MLL0 CT bands. Similarly, the broad band between 450 and 575 nm in the spectrum of 3 is assigned as a 3MLL0 CT transition based on the ADF calculations, increased in intensity relative to the Ru analogue because of the larger spinorbit coupling in Os. In 1, the shoulder at 320 nm was assigned to the terthienyl π f π* transition. This blue-shifts in 3, and is almost completely hidden under the bpy transition, and a small shoulder can be seen at ∼300 nm. The spectra of P,C complexes 2 and 4 contain more peaks than the corresponding P,S complexes 1 and 3. The high energy peaks (λ < 375 nm, assigned to bpy and terthienyl based ππ* transitions) do not shift much between 2 and 4. A band at 5116

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Figure 4. UVvis spectra of (a) PT3, (b) 1, (c) 3, (d) 2, and (e) 4 in CH3CN after 0, 2, 5, 15, 30, 60, 90, and 120 min of irradiation at 366 nm.

400 nm is observed in the spectrum of 4 which is not present in the spectrum of 2; it is not clear what the origin of this new peak is. The charge transfer transition in 4 is red-shifted relative to the corresponding band in 2. The 3MLL0 CT transition, that ADF calculations suggest corresponds to a HOMO and/or HOMO-1 to LUMO transition, red shifts to 650 nm, with the addition of a new shoulder at 560 nm. This red shift is consistent with the lower calculated atomic charge on the metal in 4 relative to that in 2. There is a significant red-shift of the terthienyl band between the P,S and P,C complexes because of either the increased planarity of the thiophene rings in the P,C bound complexes or the change in the charge of the PT3 ligand accompanying the orthometalation to form the P,C derivatives. Photostability studies of PT3 and the complexes in CH3CN were carried out by extended irradiation with 366 nm light, using UV/vis spectroscopy as a tool to probe stability. Irradiation of PT3 resulted in a decrease in the main absorption bands, and the growth of a low energy shoulder at ∼450 nm (Figure 4a). Previous studies have shown that irradiation of 2,20 :50 200 -terthiophene (T3) with UV light results in polymerization to give longer oligomers.27 The low energy absorption band observed upon irradiation of PT3 is consistent with oligomerization of the terthiophene, resulting in a red shift in the absorption band. Irradiation of complexes 2, 3, and 4 under the same conditions results in only very small changes to the UV/vis spectra (Figure 4ce), indicating that these complexes are all photostable under the irradiation conditions. On the contrary, irradiation of complex 1 results in substantial changes in the UV/vis spectrum (Figure 4b), demonstrating that this complex is not

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photostable under these conditions in solution. Although a small low energy shoulder also appears in this experiment, the higher energy region shows different changes than observed when PT3 is irradiated, suggesting that different products are formed in the case of 1. Mass spectroscopic analysis of a solution of photoirradiated 1 suggests that a bipyridyl group is being lost during the irradiation process. This has been observed before in other photoactive metal bipyridyl complexes.28 The photostability of the complexes did not change when irradiated under nitrogen or oxygen. Decomposition reactions of other metal bipyridyl complexes have been observed when MC states are accessible.29 The increased photostability of the P,C bound species may be due to the reactive MC state lying higher in energy because of the coordination of the formally anionic ligand. The higher photostability of the Os complex 3 may also be due to a higher barrier to the MC state, since the lowest energy CT excited state in this complex is lower in energy than in 1. Emission Spectra. PT3 has a short-lived emission centered at 435 nm (Table 1), attributed to radiative decay of the singlet ππ* state. Complexes 1 and 2 were previously reported to be either non-emissive or very weakly emissive upon excitation with visible light.15 However, when excited at 355 nm, complex 1 exhibited short-lived emission at 430 nm.30 The Os analogue 3 showed dual emission, with bands at 447 and 640 nm. Dual emission from metal complexes is unusual, but has been previously observed in some cases.31 In both 1 and 3, the lifetime of the species giving rise to the higher energy band is not sensitive to the presence of oxygen. The lifetime of the species giving rise to the lower energy band in 3 is significantly shorter in oxygen sparged solution. On the basis of these observations, the higher energy band in both 1 and 3 is assigned to emission from a terthiophene-localized singlet state. In 3, enhanced intersystem crossing because of the heavier Os center also populates either a 3 MLL0 CT or 3LL0 CT state which emits at lower energy, and has a longer lifetime. In complexes 2 and 4, two emission bands are also observed. In the presence of O2 the lower energy band becomes very weak in these complexes, and in 4 the lifetime is shorter in the presence of oxygen, supporting the assignment of this band to a triplet emission. Despite careful efforts to purify the complexes, the possibility of the higher energy emission band arising from free ligand cannot be entirely ruled out. However, emission of PT3 occurs with λmax = 435 nm, and the higher energy emission band observed in all the complexes is shifted from this maximum. Quantum yields were obtained for the lower energy emission band, and are comparable to those reported for related Os and Ru complexes.32,33 Values were calculated for the radiative (kr) and nonradiative (knr) decay constants and show the nonradiative decay dominates. Exciting complexes 14 at longer wavelengths resulted in only the lower energy emission being observed. Transient Absorption. Transient absorption (TA) spectroscopy uses an excitation pulse to promote a fraction of molecules into the excited state and measures the difference in absorption between the ground state and excited state. The bands in a TA spectrum indicate an excited state absorption, while bleaches are due to stronger ground state absorptions than that of the excited state. The TA spectrum of PT3 is shown in Figure 5. This spectrum shows a transient species absorbing between 400 and 600 nm, with a lifetime of 9 μs under N2, that is quenched under O2. Excitation of T3 also results in a species with a similar, but sharper, absorbance from 400 to 600 nm with a 2.8 μs lifetime (Supporting Information, Figure S8). Under oxygen, the TA 5117

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Table 1. Photophysical Data for PT3 and Complexes 14 τema

τTAa E1/2 ox ( 0.01 V E1/2 red ( 0.01 V

compound

λema

N2 sparged O2 sparged N2 sparged O2 sparged

PT3

435 nm

2.00σ(I)) = 0.030, wR2 (I > 2.00σ(I)) = 0.061. Computational Methods. DFT calculations were performed with the 2009.01 version of the Amsterdam Density Functional (ADF) program.58 Experimental X-ray crystallographic cation geometries (C1 point group) were used for all four complexes. All calculations included scalar and spinorbit relativistic effects through the zeroth-order relativistic approximation (ZORA),59 except for bond orders and natural population analysis (NPA) atomic charges60 which included only scalar relativistic effects. The generalized gradient approximation (GGA) approximation of DFT at the BP86 level was used in all cases,58k,l and all-electron (i.e., no frozen core approximation was applied) TZ2P basis sets from the ADF basis sets ZORA library were used for all atoms. Atomic charges on all atoms were calculated within ADF using the Voronoi (VDD),61 Hirshfeld,62 Bader atoms in molecule (AIM),63 NPA, Mulliken, and multipole-derived quadrupole (MDC-q) methods. Bond orders were calculated within ADF using the Mayer, Gophinatan Jug, and three NalejawskiMrozek methods N-M (1), N-M (2), and N-M (3) provided with the ADF program.64 5120

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’ ASSOCIATED CONTENT

bS

Supporting Information. X-ray crystallographic data in CIF format for 3 and 4, 1H NMR spectra for 3 and 4, selected bond lengths and angles for complexes 14, cyclic voltammograms for 3 and 4, TA spectra of T3, ADF orbital plots of 1 and 2, and tables of ADF-calculated total metal bond orders and metal atomic charges. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We thank the Natural Sciences and Engineering Research Council of Canada for funding, the Laboratory for Advanced Spectroscopy and Imaging (LASIR) for access to spectroscopic equipment, Dr. Saeid Kamal for assistance with the TA measurements. S.M. thanks NSERC, UBC, and the Walter C. Sumner Foundation for graduate fellowships. ’ REFERENCES (1) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes, 1st ed.; Academic Press; London, 1992. (2) Anderson, P. A.; Keene, F. R.; Meyer, T. J.; Moss, J. A.; Strouse, G. F.; Treadway, J. A. J. Chem. Soc., Dalton Trans. 2002, 3820–3831. (3) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737–740. (4) Liang, Y.; Feng, D.; Guo, J.; Szarko, J. M.; Ray, C.; Chen, L. X.; Yu, L. Macromolecules 2009, 42, 1091–1098. (5) Howard, I. A.; Laquai, F.; Keivanidis, P. E.; Friend, R. H.; Greenham, N. C. J. Phys. Chem. C. 2009, 113, 21225–21232. (6) Clarke, T. M.; Durrant, J. R. Chem. Rev. 2010, 110, 6736 6767. (7) Walters, K. A.; Ley, K. D.; Cavalaheiro, C. S. P.; Miller, S. E.; Gosztola, D.; Wasielewski, M. R.; Bussandri, A. P.; van Willigen, H.; Schanze, K. S. J. Am. Chem. Soc. 2001, 123, 8329–8342. (8) Meyer, T. J. Pure Appl. Chem. 1986, 58, 1193–1206. (9) (a) Wong, C. T.; Chan, W. K. Adv. Mater. 1999, 11, 455–459. (b) Wong, W.-Y.; Wang, X.-Z.; He, Z.; Djurisic, A. B.; Yip, C.-T.; Cheung, K.-Y.; Wang, H.; Mak, C. S. K.; Chan, W.-K. Nat. Mater. 2007, 6, 521–527. (c) Wong, W.-Y.; Wang, X.-Z.; He, Z.; Chan, K.-K.; Djurisic, A. B.; Cheung, K.-Y.; Yip, C.-T.; Ng, A. M-C.; Xi, Y. Y.; Mak, C. S. K.; Chan, W.-K. J. Am. Chem. Soc. 2007, 129, 14372–14380. (d) Wong, W.Y.; Ho, C.-L. Acc. Chem. Res. 2010, 43, 1246–1256. (e) Liu, L.; Ho, C.-L.; Wong, W.-Y.; Cheung, K.-Y.; Fung, M.-K.; Lam, W.-T.; Djurisic, A. B.; Chan, W.-K. Adv. Funct. Mater. 2008, 18, 2824–2833. (10) Zhu, Y.; Millet, D. B.; Wolf, M. O.; Rettig, S. J. Organometallics 1999, 18, 1930–1938. (11) Weinberger, D. A.; Higgins, T. B.; Mirkin, C. A.; Stern, C. L.; Liable-Sands, L. M.; Rheingold, A. L. J. Am. Chem. Soc. 2001, 123, 2503– 2516. (12) Zhu, S. S.; Swager, T. M. Adv. Mater. 1996, 8, 497–500. (13) Higgins, T. B.; Mirkin, C. A. Chem. Mater. 1998, 10, 1589–1595. (14) Moorlag, C.; Clot, O.; Wolf, M. O.; Patrick, B. O. Chem. Commun. 2002, 3028–3029. (15) Moorlag, C.; Wolf, M. O.; Bohne, C.; Patrick, B. O. J. Am. Chem. Soc. 2005, 127, 6382–6393. (16) Moorlag, C.; Sarkar, B.; Sanrame, C. N.; B€auerle, P.; Kaim, W.; Wolf, M. O. Inorg. Chem. 2006, 45, 7044–7046. (17) Mayo, E. I.; Kilsa, K.; Tirrell, T.; Djurovich, P. I.; Tamayo, A.; Thompson, M. E.; Lewis, N. S.; Gray, H. B. Photochem. Photobiol. Sci. 2006, 5, 871–873.

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